1. Field of the Invention
The present invention relates to rotatable variable resistors provided with clicking mechanisms, and particularly, to a rotatable variable resistor provided with a clicking mechanism used in, for example, in-vehicle meters for controlling light intensity.
2. Description of the Related Art
In known rotatable variable resistors, a wiper fixed to a rotor slides on a resistor substrate in a circular-arc manner so that the resistance value is varied with respect to the rotational angle of the rotor. Japanese Unexamined Patent Application Publication No. S62-189704 discloses a rotatable variable resistor which includes a wiper and a contact substrate. The wiper is provided with a clicking projection, and the contact substrate is provided with a clicking depression. The clicking projection and the clicking depression are disposed in positions having the same radius, along which the clicking projection slides. With the rotation of the wiper, the clicking projection slides on the contact substrate and engages with the clicking depression, thus imparting a feel of a “click” to an operator.
FIGS. 13 and 14 illustrate an example of a known rotatable variable resistor 100 provided with a clicking mechanism, which includes a wiper 128 provided with a semispherical clicking-projection 129a and a semi-oval-shaped non-clicking-projection 129b. 
The variable resistor 100 further includes a rotor 118 which is integrated with and fixed to the wiper 128. The rotor 118 is rotatably supported by an insulative casing 102 and a collector 133 such that the rotor 118 can rotate within a predetermined angle. A resistor layer 142 is provided on a resistor substrate 141 which is fixed to the casing 102. The wiper 128 has contact portions 130 and 131 which are resiliently urged against the resistor layer 142. By rotating the rotor 118, the contact portions 130 and 131 slide on the resistor layer 142.
The resistor layer 142 is electrically connected to two external terminals 111 and 112 which are fixed to the casing 102. The collector 133 is provided with a clicking depression 134 and an external terminal 135. Moreover, the collector 133 is resiliently urged against the wiper 128 and is electrically connected with the wiper 128.
As described above, the wiper 128 is provided with the clicking projection 129a and the non-clicking projection 129b, and the collector 133 is provided with the clicking depression 134. The two projections 129a and 129b are resiliently urged against the collector 133 in a slidable manner. The clicking projection 129a slides on the collector 133 and engages with the clicking depression 134 to impart a feel of a “click” to an operator. On the other hand, the top portion of the non-clicking projection 129b is made slightly oblong in the sliding direction so as to prevent the projection 129b from engaging with the clicking depression 134.
The reasons for providing the non-clicking projection 129b are as follows:
1. By providing the non-clicking projection 129b in a substantially symmetrical position with respect to the clicking projection 129a and constructing the projection 129b to have the same height as the projection 129a, the balance of the wiper 128 and the rotor 118 are maintained, thereby preventing the wiper 128 and the rotor 118 from tilting.
2. Although the enter/exit action of the clicking projection 129a with the clicking depression 134 can cause unsteady contact between the wiper 128 and the collector 133, the non-clicking projection 129b stabilizes the contact between the two.
In this conventional variable resistor 100, however, since the top portions of the clicking projection 129a and non-clicking projection 129b have the same rotational radius, the following problems exist.
Since tilting of the rotor 118 cannot be prevented due to the reaction force of the wiper 128, the top portion of the non-clicking projection 129b in that case may engage with the depression 134 to impart an undesired feel of a “click”, thus deteriorating the operational feel of the variable resistor 100.
Furthermore, when the two projections 129a and 129b slide on the collector 133, abrasion dust is released and deposited in the ends of their sliding tracks. The projections 129a and 129b slide over each other's abrasion dust and generate a noise, which may cause disturbance of the output.
Furthermore, the use of the casing 102 in the variable resistor 100 increases the number of components and thus, complicates the assembly process.